The present invention relates to an electrophotographic liquid developer for the reversal devleopment of negatively charged charge images, comprising an insulating carrier liquid of high electrical resistivity and a low dielectric constant, in which a pigment, a resinous binder, a charge controller comprising a copolymer of N-vinyl-2-pyrrolidone and a methacrylic acid ester, and conventional additives are dispersed or dissolved, and to a process for the preparation thereof.
Published European Application No. 0,001,103 (=U.S. Pat. No. 4,243,736) discloses a liquid developer containing negatively charged toner particles for direct development, i.e., for the development of positively charged electrostatic charge images, which comprises, in an electrically insulating carrier liquid, a pigment, a resinous binder, and, as the charge controller, a copolymer of N-vinyl-2-pyrrolidone and a methacrylic acid ester, either as a dispersion or as a solution. This liquid developer is, however, not adjusted to, nor suitable for, the reversal development of negatively charged charge images, for copies prepared using this developer in general show strong scumming and result in unsatisfactory image reproductions.
U.S. Pat. No. 3,542,681 discloses a negative-working electrostatic toner composition for producing positive images and reversal images, wherein a pigment and an alkylized polymer of a heterocyclic N-vinyl monomer, such as N-vinyl-2-pyrrolidone, are dispersed in an electrically insulating carrier liquid. It has been found, however, that the quality of images produced using liquid developers of this type is not satisfactory, especially in the case of reversal development. Therefore, German Offenlegungsschrift No. 1,930,783, which is based on U.S. Pat. No. 3,542,681, proposes a liquid developer containing a controlling agent, but which is suitable for a positive-working developing process only. Comparative tests have shown that this liquid developer does not yield copies of satisfactory quality when it is used for reversal development of negatively charged charge images.
In German Auslegeschrift No. 2,125,668 (=U.S. Pat. No. 3,674,693), a liquid developer for the reversal development of negatively charged charge images is described, which developer comprises a mixture of a pigment, a polycyclopentadiene resin and a phospholipid as controller, dispersed in an aliphatic carrier liquid. However, charge control by means of, for example, lecithin, cannot always be adjusted precisely enough to ensure reproducible results.